If you can get access to one of those laser thermal digital guns, warm up the car to normal and check the temp of the pipe where it goes in and comes out of the Cat. If flowing and working the temps will be two to three times higher on the output of the converter. If restricted, the temps will be higher on the input side and the same or cooler on the output side.
If you decide it is plugged you can take it off and shove a lenght of rebar and just chip out all the element till you can see through it then bolt it back on. (edited)